A while back, I wrote a Firefox extension that converts pinyin with tone numbers into pinyin with tone marks above the appropriate vowels. It works on MS/Mac/Unix and it's a button that sits in your browser's Add-on bar, so it won't get in your way like an IME would when you don't want it.

For example, if you type “zhong1wen2″ into any plain-text field* in your browser, highlight it and hit the button, then it will be converted into “zhōngwén”. It can also handle conversions of pinyin words mixed into English sentences, such as

Zhu1 Ge3liang4 and Cao2cao1 were famous generals. -> Zhū Gěliàng and Cáocāo were famous generals.

The specifics of the conversion process are identical to those of the online pinyin converter on Toshuo.com, which is also powers the SinoSplice Tooltips plugin for WordPress. There's a quick youtube video showing its usage here: http://www.youtube.com/watch?v=i_ZOUHzjru0"]http://www.youtube.c...h?v=i_ZOUHzjru0